Illumination systems relying on light-emitting diodes (LEDs) as light sources should maintain a consistent light-illumination (i.e., intensity) level and output color coordinates (e.g., a specific set or range of x-y coordinates on the CIE Chromaticity Diagram) throughout their lifespan, even while operating in changing environmental conditions. Such consistency should not require external intervention by a user, as such intervention is generally impractical. The consistency in light output is even more important for systems assembled from many discrete illumination elements in a tiled or overlapping fashion, such as backlight units for liquid-crystal displays, as such systems should have the same illumination properties regardless of location.
In order to help assure consistent light output from illumination units or systems including multiple LEDs, manufacturers often rely upon the “binning” of LEDs into groups having substantially similar emission properties. Binning helps to reduce the amount of device-to-device variation, since it partially compensates for manufacturing differences among LEDs. However, binning is imperfect, time-consuming, and expensive, particularly when LEDs must be binned according to both intensity and emission wavelength (i.e., color).
In order to supply illumination systems and devices with consistent light-emission properties, there is a need for illumination units that are independently controllable, i.e., that incorporate sensors that detect illumination characteristics, as well as circuitry to control each LED's operation based at least in part on the sensed characteristics. Such units should account for not only short-term changes in illumination behavior (e.g., due to local temperature variation), but also longer-term changes due to, e.g., aging of the LEDs. Furthermore, since it may be desirable for each individual illumination unit to incorporate one or more sensors, such sensor(s) should interfere with light propagation from the LEDs as little as possible.